Welcome Mason, I think that you killed the goose that layed the golden egg...
7.3 km/l at 140 km/h is probably unheard of & you want to better that ?
I can understand this with a 1000cc turbo etc but not with a 4L V6

"en hul se my sensors op exhaust het weg gebrand oor di free flow"

I think that if the free flow was the cause of the sensors burning away then it will keep happening, even if you replace the sensors...
You do not say what the milage is on the vehicle, or how long after the free fow was fitted, the sensors burned off.
This could all be applicable in the diagnosis to get you the amswers you are looking for...
I have not heard of anyone getting 7.3 km/l at these speeds, so I hope that you can get your truck back to this.
Good luck...